It was probably to prevent getting confused for scripts/stylesheets when it's moved. There are a lot of code script/stylesheets/modules in every pages. They could be imported to the other wikis. That's the reason why they don't allow to do that for regular users.

I think this was changed before JS Review existed (before 2015), when the codeeditor group was a thing and people could freely modify scripts that were not protected to codeeditor-only, so it would be logical that moving scripts would require codeeditor permissions. I can ask around if there are good reasons to keep that restriction now.